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Lesson 10.2ShapeChapter 10 · Composites

Resin transfer molding and infusion

Dry fiber in a mold; liquid resin pushed in (RTM) or pulled in by vacuum (infusion)

Resin transfer molding (RTM) and resin infusion both start with dry fiber cloth laid in a mold. Liquid resin is then pushed in under pressure (RTM) or pulled in by vacuum (infusion) until every fiber is soaked. Boat hulls, wind-turbine blades and many car body parts are made this way, without costly prepreg or an autoclave.

Dry fiber goes in first, then liquid resin is pushed or pulled through itTwo cross-sections: a rigid closed mold (RTM) and a vacuum bag (infusion); same step numbersResin transfer molding (RTM): resin pushed into a closed moldresininjectorflow front →clamping pressvent: air outwet fiberdry fiber(preform)heating channels1234Vacuum infusion: resin pulled under a bag by vacuumresin potflow front →vacuumpumpoutside air presses (about 1 bar)wet fibervacuum bagflow meshdry fibercures at room temperature or in an oven1234Fiber (the part)Liquid resinMold, bag, machinesHeat
RTM and vacuum infusion compared: two cross-sections, same four stepsBoth panels share the circled numbers of the steps below. Shaded fiber is already soaked with resin; the hatched fiber ahead of the flow front is still dry.

How it works

  1. Lay in the dry fiber. Fabric layers go into the mold, often pre-shaped into a preform (a dry fiber blank that already has the part's shape). Foam or balsa cores can be added for stiff, light sandwich panels.

  2. Close the mold. In RTM, a rigid second mold half closes on top and a press clamps it shut. In infusion, a flexible plastic bag is taped over a one-sided mold and the air is pumped out.

  3. Bring in the resin. RTM pumps mixed resin (resin plus hardener) in at typically 1–10 bar, and at much higher pressure in high-pressure RTM for cars. In infusion, the vacuum pulls resin from an open pot, helped by a flow mesh (a coarse net that spreads it quickly).

  4. Fill, cure and demold. The resin creeps forward as a flow front, pushing air out through the vent or vacuum outlet. Once resin reaches the outlet, the inlet is clamped shut. The resin cures (hardens by a chemical reaction) in a heated mold, at room temperature or in an oven; then the part is released and trimmed.

Key facts

Materials
Glass, carbon or aramid (a tough synthetic fiber) fabrics; polyester, vinyl ester or epoxy resins; foam or balsa cores
Typical parts
RTM: car roofs and body parts, bike frames, jet-engine fan blades; infusion: boat hulls and decks, wind-turbine blades
Part size
RTM: palm-sized up to about car-roof size; infusion: up to boat hulls and wind blades over 100 m long
Precision
RTM: both faces molded, typically within about ±0.2–0.5 mm; infusion: one accurate face, while thickness can vary by roughly 5–10 %
Surface finish
RTM: smooth on both faces; infusion: smooth on the mold face, textured on the bag face
Batch size it suits
Infusion: one-offs to a few hundred; RTM: hundreds to tens of thousands a year
Tooling and machine cost
Infusion: one mold plus throwaway bag materials, from thousands of dollars (small parts) to millions (wind-blade molds); RTM: two matched molds plus an injection unit and press, from tens of thousands to millions
Speed
High-pressure RTM: a few minutes per part; ordinary RTM: roughly 15 minutes to a few hours; infusion: hours to a day, including bagging

Strengths and limits

Strengths

  • No prepreg, freezer or autoclave: lower material and equipment cost
  • RTM gives two finished faces and repeatable thickness
  • Infusion scales to huge parts such as hulls and blades
  • Closed process: far fewer fumes than open hand layup

Limits

  • Resin must reach every corner before it gels (starts to set), or dry spots remain
  • RTM molds must resist the injection pressure, so they are heavy and costly
  • Infusion gives only one good face and uses a lot of throwaway material
  • Fiber content and quality are usually a little lower than with autoclave prepreg

Where it's used

  • MarineBoat hulls, decks and superstructures
  • Wind energyWind-turbine blades, the largest over 100 m long
  • AutomotiveCarbon roof panels, body and chassis parts (high-pressure RTM)
  • AerospaceJet-engine fan blades from 3D-woven carbon, brackets, doors and frames
  • SportsCarbon bike frames and forks, kayaks

How to recognize it

  • RTM parts are smooth on both faces, with a thin trimmed edge where the two mold halves met.
  • Infused parts are glossy on the mold side and show a faint mesh or peel-ply texture on the inside face.
  • Small stubs or ground-off spots where the resin inlet and the vent were attached.
  • Very large one-piece shells, like hulls and blades, with no joints in the skin.

Compare with

Layup and autoclave curing10.1the resin arrives already inside the fiber (prepreg) and cures under high pressure; better quality, much higher cost.Compression molding2.6a premixed fiber-and-resin charge is squeezed in a hot press; faster for small parts, less control of where fibers go.Injection molding2.1also fills a closed mold, but with molten plastic carrying at most short chopped fibers, not long fabric.

Watch it

SAERTEX Group · A short explainer of the RTM steps

Easy Composites Ltd · Bagging, flow mesh and the moving flow front, start to finish

TPI Composites · Infusion at giant scale in a wind-blade factory

TIME Bicycles · RTM used to make carbon bike frames

See real photos

Remember

  • Dry fiber goes in first; liquid resin follows.
  • RTM pushes resin into a rigid closed mold; infusion pulls it under a bag with vacuum.
  • Cheaper than prepreg and autoclave, and big enough for hulls and wind blades.

More in Composites

All 100 processes
10.1Layup and autoclave curingPrepreg plies stacked on a mold, vacuum-bagged and cured under heat and pressureOpen the lesson10.3PultrusionFibers pulled through resin and a heated die, nonstopOpen the lesson10.4Filament windingResin-wet fiber wound onto a spinning mandrelOpen the lesson

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